Hi Marco,
As far as I understood the problem you describe is that the panes do not preserve their proportional size when they have been collapsed, you resize the browser and then expand them. If so, this behavior is the expected one because of the way the splitter works. When you collapse a RadPane, it "remembers" its size before it is collpased and when you expand it later it shows with this "old" size. We cannot expand it with the proportional size because there are various scenarios which will break in this case - e.g if other size has been set through client code or if the user resizes the RadPane by using the RadSplitBar before collapsing it, etc. This being said, keeping the proportions after the pane has been collapsed and the browser resized is correct only if all the RadPanes sizes are set in percentages and there are not any RadSplitBars which is not the common case.
I suggest to calculate the size of the RadPane and set it in the OnClientExpanded event e.g as shwon below:
Please, use this idea as a start point and direction and extend it further in order to fit in your particular case.
All the best,
Svetlina
the Telerik team